Hydraulic fracturing has been widely used as a means for improving the rates at which fluids can be injected into or withdrawn from subterranean formations surrounding oil wells, gas wells, water injection wells, and similar boreholes. The fracturing methods employed normally involve the injection of a viscous fracturing fluid having a low fluid loss value into the well at a rate and pressure sufficient to generate a fracture in the exposed formation, the introduction of fluid containing suspended propping agent particles into the resultant fracture, and the subsequent shutting in of the well until the formation is closed on the injected particles. This results in the formation of a vertical, high-conductivity channels through which fluids can thereafter be injected or produced. The conductivity obtained is a function of the fracture dimensions and the permeability of the bed of propping agent particles within the fracture.
In order to generate the fracture of sufficient length and width and to carry the propping agent particles into the fracture, it is necessary for the fluid to have relatively high viscosity, low friction loss, and non-damaging properties. The viscosity in aqueous liquids is provided by the addition of polymers, frequently called thickeners. Following the treatment of the well, it is desirable to return the aqueous liquids to its low viscosity state to enhance cleanup, thereby permitting the fracturing fluid and polymer to be removed from the formation and the propped fracture. The highly viscous liquid if left in the fracture would reduce formation permeability and impede the production of formation fluids through the propped fracture. Moreover, the residue of the polymer on the fracture face and in the pores of the propped fracture would significantly reduce fluid permeability therethrough.
The polymers used as aqueous thickeners must impart sufficient fluid viscosity at the temperature of the formation to generate the desired fracture and suspend the proppant. The polymers are frequently crosslinked to achieve the necessary viscosity.
In order to avoid the undesirable after effects of the polymer and polymer residue, it is now common practice to employ in the fracturing fluid chemicals (“breaker”) which degrade the polymers. U.S. Pat. No. 4,741,401 discloses a number of oxidizing agents contained in capsules for breaking the fracture fluid. U.S. Pat. No. 3,938,594 discloses the use of sodium hypochlorite solution, acid, micellar solutions, and surfactants for degrading the fracturing fluid polymers.
In gravel packing operations, solid gravel particles such as sand are carried to the subterranean zone or formation in which a gravel pack is to be placed by a high viscosity crosslinked gelled fluid. That is, the gravel is suspended in the high viscosity fluid at the surface and carried to the subterranean zone or formation in which the gravel pack is to be placed. Once the gravel is placed in the zone or formation, the crosslinked gel is broken (degraded) and returned to the surface. The gravel pack produced functions as a filter to separate formation solids from produced fluids while permitting the produced fluids to flow into and up the wellbore.
The polymers and crosslinking agents used in well treating fluids are well known in the art. Typical hydratable, water-soluble polymers which can be crosslinked are the galactomannan gums, glucomannan gums, guars, derivatized guars and cellulose derivatives. Specific examples are guar gum, guar gum derivatives, locust bean gum, tara gum, karaya gum, cassia gum, carboxymethyl cellulose, carboxymethylhydroxyethyl cellulose and hydroxyethyl cellulose. A suitable synthetic polymer is polyvinyl alcohol.
A variety of crosslinking agents have been utilized for crosslinking a polysaccharide gelled aqueous liquid, particularly suitable such crosslinking agents are transition metal containing compounds which release transition metal ions when dissolved in an aqueous liquid and borate releasing compounds. Examples of particularly suitable transition metal ions for crosslinking the polymer gelled aqueous liquids described above are titanium IV (4+), zirconium IV (4+), antimony III (3+), chromium III (3+) and aluminum III (3+). Examples of compounds which are water soluble and which supply zirconium IV ions are zirconium lactate, zirconium carbonate, zirconium acetylacetonate and zirconium diisopropylamine lactate. Compounds capable of supplying titanium IV, antimony III, chromium III and aluminum III are well known to those skilled in the art and comprise similarly conventional compounds such as potassium pyroantimonate, titanium acetylacetonate, titanium triethanolamine, chromium III citrate, aluminum acetate and the like. A borate releasing compound has also been utilized as a crosslinking agent. The particular borate compound used may be any compound which supplies borate ions in a hydrated polysaccharide gelled aqueous liquid. For example, the borate source may be a rapidly soluble borate containing compound such as boric acid, borax or “POLYBOR” manufactured by the U.S. Borax Company. The borate source may also be a slowly soluble borate such as alkaline earth metal borates, alkali metal borates and the like. The use of slowly soluble borate releasing compounds in a gelled aqueous treating fluid delays a significant viscosity increase due to crosslinking until after the treating fluid is pumped into the well bore. The borate releasing compounds may either by hydrated or anhydrous.
See for example the following U.S. patents, incorporated herein by reference: Wadhwa U.S. Pat. No. 4,519,309; Mondshine U.S. Pat. No. 4,619,776; Dawson U.S. Pat. No. 5,145,590; Sharif U.S. Pat. No. 5,160,445; Sharif U.S. Pat. No. 5,252,236; Sharif U.S. Pat. No. 5,266,224; Sharif U.S. Pat. No. 5,310,489; Kinsey U.S. Pat. No. 5,488,083; Kinsey U.S. Pat. No. 5,565,513; Shuchart U.S. Pat. No. 5,759,964; Moorhouse U.S. Pat. No. 6,225,264; and Moorhouse 6,251,838.
It is known to provide the polymer crosslinking agents in the form of a concentrate suspended in an appropriate liquid suspension medium. Thus crosslinking agents have been suspended in aqueous liquids and non-aqueous liquids such as a hydrocarbon such as diesel, mineral oils, and kerosene, and alcohols containing 6–12 carbon atoms, vegetable oils, ester-alcohols, polyol ethers, glycols, animal oils, silicone oils, halogenated solvents, mineral spirits-resin solutions, and oil-resin solutions. See for example U.S. Pat. No. 6,024,170.
Numerous problems exist when utilizing these concentrates. Thus many of the non-aqueous liquid suspension mediums are environmentally unacceptable and have poor suspension and stability characteristics, and many are expensive and difficult to viscosity. Aqueous based concentrates are unacceptable at low temperatures as their viscosity increases such that they become non-pourable or solidify.